Imagine that you have a sample of a drum loop that you want to use in a track you are working on. The loop is 108 bpm and your track is 90 bpm. What do you do? You can of course pitch down the loop, but that will make the loop sound very different, and what if there are harmonies in the loop that you want to match to your song. You can also time stretch it. That will keep the pitch, but will make the loop sound different. Usually it means that you loose some “punch” in the loop.